Up until now, in order to obtain super high vacuums in various experimental apparatus and manufacturing equipment, a turbo molecular pump such as shown in FIG. 11 has been used.
The turbo molecular pump comprises a cylindrical housing 1, a rotating shaft 2 supported inside the cylindrical housing 1, a motor 3 for rotating the rotating shaft 2, a rotor 4 supported on the rotating shaft 2, a plurality of vanes 5 provided on the upper end periphery of the rotor 4, a cylindrical casing 7 enclosing the periphery of the housing 1 and having a suction port 6 at one end thereof, a plurality of stator blades 9 supported on an inner peripheral face of the casing 7 which together with the rotor vanes 5 make up a pump 8, and a discharge port 10 for discharging the air drawn in through the suction port 6 by means of the pump 8.
An upper magnetic ring 11 and lower magnetic ring 12 are fixedly fitted at respective upper and lower locations on the outer peripheral surface of the rotating shaft 2. An upper radial electromagnet 13 is provided on an inner periphery of the upper portion of the housing 1 in a position opposite the upper magnetic ring 11, thereby making up the upper radial magnetic bearing 14. Furthermore, a lower radial electromagnet 15 is provided on an inner periphery of the lower portion of the housing 1 in a position opposite the lower magnetic ring 12, thereby making up the lower radial magnetic bearing 16.
Furthermore, a flange 20 of a magnetic material is provided on a central outer periphery of the rotating shaft 2, and a pair of upper and lower thrust electromagnets 21 are supportingly fixed to an inner face of the housing 1 and opposing the magnetic flange 20, thereby forming a thrust magnet bearing 22.
The position of the rotating shaft in a radial direction is detected by upper and lower radial sensors 17 and 18 provided at respective upper and lower positions on the inner face of the housing 1. Similarly, the position of the rotating shaft 2 in the thrust direction is detected by a thrust sensor 19 provided between a lower end face of the rotating shaft 2 and the bottom face of the housing 1.
Signal indicating the detected values from the respective sensors 17 to 19 are input to a controller (not shown in the figure) which controls the power to the electromagnets 13, 15 and 21 based on the signals from the respective sensors 17 to 19 so as to maintain the rotating shaft 2 in a floating condition. As a result, the rotating shaft 2 is able to rotate at super high speed without any contact of parts.
In operating the conventional turbo molecular pump constructed above, the rotating shaft is maintained in a floating condition by means of signals from the controller, and the motor 3 is switched on. The rotating shaft 2 and the rotor 4 then rotate up to high speed, depending on the power supplied to the motor 3, and air sucked in from the inlet port 6 by the pump 8 comprising drive vanes 5 and stator blades 9, is discharged from the discharge port 10, so that components connected to the suction port 6 can be held at a super high vacuum condition.
Touch-down bearings 23 are provided for preventing severe rubbing between members rotating with the rotating shaft 2 and the members fixed to the housing 1 when the power to the respective electromagnets 13, 15, 21 is stopped at the time of power cuts etc. In addition, a connector is provided for supplying power to the respective electromagnets 13, 15, 21 and for outputting the detected signals from the respective sensors 17 to 19.
With the conventional turbo molecular pump constructed and operated as above, since controllable type magnetic bearings 14, 16, 22 are used to support the rotating shaft 2 in a floating condition, it is difficult to avoid high manufacturing costs due to the complicated nature of the construction.
With this construction, since the floating condition of the rotating shaft 2 is maintained by control of the power to the electromagnets 13, 15, 21, then precise sensors having very high response characteristics are required for the sensors 17 to 19. Furthermore, the controller for controlling the power to the respective electromagnets 13, 15, 21, based on the signals from the respective sensors 17 to 19, must also have exceptionally high response characteristics. As a consequence manufacturing costs are increased.
To deal with this problem, a bearing unit is developed in which a rotating shaft with a permanent magnet supportingly fixed thereto is maintained in a floating condition by utilizing the repulsive force between a superconducting body and permanent magnet based on the Meissner effect. Research with this so-called superconducting bearing unit is progressing, however the force for floating the rotating shaft is small, so that it is not possible to support a heavily loaded rotating shaft.
This is because the threshold of the magnetic field wherein the Meissner effect occurs is extremely smaLL. For example, when using YBa.sub.2 Cu.sub.3 O.sub.7 for the super conducting body, this has a magnetic intensity of around 200 Oersted when cooled in liquid nitrogen (77 degrees K). Consequently, when the rotating shaft is floated by means of the Meissner effect, the weight of the rotating shaft can be no more than 1 Kg for a practical size of superconducting bearing unit.
On the other hand, with recent research (see, for example, "Cryogenics Engineering Journal" Vol.26. 1991, Chapter 26 page 70), success had been achieved with superconducting materials of YBaCuO compounds in inducing a pinning point (the point existing in superconducting materials at which a pinning force occurs). The pinning force occurs as a result of a screening or shielding current flowing within the superconducting body, which acts to restrict those magnetic flux of force generated by the permanent magnet that penetrate into the superconducting body. The pinning force acts in a direction to prevent a change in distance between the superconducting body and the permanent magnet when this distance is changed. That is to say, a repulsive force acts between the superconducting body and the permanent magnet when the superconducting body tends to approach the permanent magnet, and an attractive force acts between the superconducting body and the permanent magnet when the superconducting body tends to separate from the permanent magnet.
The repulsive force attributable to the pinning force as described above, is much larger than the repulsive force due to the Meissner effect, and by appropriate selection of the permanent magnet, a force of approximately 10 N/cm.sup.2 can be obtained. Furthermore, there is a new operating force in the form of an attractive force attributable to a pinning effect which has notoccurred in superconducting bodies used heretofore. With this force, as with the above-mentioned repulsive force, a force of approximately 10 N/cm.sup.2 can be obtained by suitable selection of the permanent magnet. Accordingly, if a superconducting bearing unit is made utilizing this pinning force, a rotating shaft of a practical size and built-up weight can be supported.
It should be noted that although superconducting bearing units conventionally proposed have been constructed so that the rotating shaft is floated by utilizing the repulsive force due to the Meissner effect, constructions wherein the rotating shaft is supported by a working force attributable to the pinning force (attractive force and repulsive force) do not exist in the prior art.